The invention relates generally to integrated optical circuits and components making up such circuits, and more particularly, to low loss single mode channel waveguides that operate at 1.3 to 1.6 micron wavelengths, or at longer wavelengths. The recent development of low loss single mode optical ibers with low dispersion at the 1.3 or 1.5 micron wavelengths has focussed attention on long wave integrated optical circuits and optical systems that couple to such fibers. Such optical circuits and systems are useful in telecommunication, data communication, optical signal processing, optical interconnection, optical sensing, and microwave antenna control applications. Semiconductor waveguides circuits are of special interest because they can provide opto-electronic integration; that is the monolithic integration of optical guided wave components with electronic circuits and with electrooptical components on a single chip.
The fundamental building block of such guided wave circuits is the channel waveguide which is used to make directional couplers, optical switches, optical modulators and optical interconnects between the various components. It is essential that optical propagation losses be kept to a minimum in such channels (less than 1 dB/cm) to allow multiple guided wave components to be cascaded on one wafer (such as in a switching network) without incurring a significant loss penalty.
Another important need is to provide channel waveguides with physically small size so that the multiple waveguides may be densely packed on a chip. In addition, the waveguide fabrication techniques should be relatively simple and should have the capability of permitting the fabrication of a wide variety of channel waveguide shapes. For example, the techniques should allow stacked three dimensional integration of waveguide components as well as planar side by side (2 dimensional) integration of components.
Two prior art waveguide fabrication techniques have been used with some success: hetero-epitaxy of III-V semiconductors and homoepitaxy of silicon on silicon.
Prior art fabrication of III-V semiconductor guided wave components for the 1.3 to 1.5 micron wavelengths suffer from the complexity of using binary, ternary, and quatenary alloy compositions of various materials, and the problems which arise from heteroepitaxy of different volatile materials upon each other. As a result, these techniques are extremely sophisticated and do nt consistently produce good quality components. Hetero-epitaxy techniques being somewhat complex, are relatively expensive to apply as a manufacturing process. The use of crystalline silicon alleviates most difficulties because the waveguide core uses only a single elementary group IV material. In the past, epitaxial silicon-on-silicon waveguides have been formed into channels by dry etching. In such a procedure, a lightly doped waveguiding layer is grown on a heavily doped substrate. The refractive index of the substrate is typically 0.01 lower than the index of the epitaxial guiding layer. The problem with such a technique is that the guided-mode light tends to leak or "tail" evanescently into the heavily doped substrate because the index step between the layers is not large enough to offer tight model confinement. Moreover, the substrate tends to cause high optical losses due to the large concentration of free carriers therein. The optical propagation loss in channels fabricated with this homojunction prior art technique is therefore in the range of 10 to 15 dB/cm. Losses in this range are unacceptable for medium scale integration of guided wave components, since accummulating losses can result in loss of optical signal. It is also impractical to shrink waveguide channels made with this technique to less than 1 micron because of the optical losses which become very large due to the extremely high doping of the silicon waveguide "cladding".
It is clearly evident that there exists a need for improved optical channel waveguides which are not subject to the loss, quality and size drawbacks associated with prior art waveguides. A need also exists for improved techniques that simplify integrated optical chip manufacturing and consistently produce high quality electrooptical chips at low cost.